1. Field of the Invention
This invention relates generally to forced air heaters and in particular to fired heaters intended for portable use.
2. Description of Related Technology
The prior art includes a variety of air heaters suitable for heating portable shelters, garages and the like. As shown in U.S. Pat. No. 2,421,370, such air heaters generally include an outer casing, a smaller dram enclosing a combustion chamber positioned within the casing to define a plurality of air heating passages extending through the heater between the periphery of the drum and the casing, and a fan secured at one end of the casing suitable for forcing venting air through the heating passages. A fuel burner is positioned at the upstream end of the drum to accommodate the delivery of an axially directed flame into the combustion chamber toward its downstream end where the resulting combustion gases are evacuated frown the chamber. When this type of burner is put into operation, the peripheral region of the combustion chamber surrounding the burner is relatively cool compared to its downstream end where the temperature within the chamber is generally in excess of 1100.degree. Celsius.
Due to this temperature differential along the length of tile combustion drum, only seventy percent of the drum's surface area is effectively utilized to heat the air moving through the casing. In order to obtain greater heat transfer from the drum to the venting air moving through the air heating passages, a series of vanes or baffles is provided at the downstream end of the drum to increase the radiant surface area exposed to the venting air flow about the periphery of the drum, or to provide a relatively large number of smaller heating passages. The solution of such three dimensional fluid flow and heat transfer problems is an inexact science, and the approaches just described have heretofore resulted in a substantial pressure drop in air flowing through the heater as well as being relatively difficult to fabricate and repair. Also, depending on the type of fuel used, such arrangements make complete combustion of the fuel difficult. As a result, carbon tends to build up on the walls of the combustion chamber, which in turn leads to corrosion and pitting in the walls, ultimately rendering the drum unserviceable.
Attempts have been made to eliminate the vanes and baffles required at the downstream end of the drum to obtain sufficient heat transfer and eliminate the associated pressure drop brought about by such vanes and baffles. In U.S. Pat. No. 4,309,978 a portable heater structure was disclosed which reversed tile combustion gas flow at the downstream end of the combustion drum to form a flow of hot combustion gases moving towards discharge ports at the upstream end of the combustion drum, thus more effectively utilizing the entire length of tile heater for heat exchange purposes and obtaining complete combustion without corrosive carbon deposits within the drum. In order to accomplish this and at the same time support the combustion drum within the outer casing, a particular type of ribbed drum was provided with the ribs abutting the casing and sometimes leading to hot spots at the areas of abutment, gathering particulate material and interfering with heat exchange flow. Further, the manufacturing and assembly steps required were often extensive, complex and hardly economical.
One recurrent theme in previous attempts to mix fuel and air within the combustion chamber, while also controlling flame parameters such as shape, size and location within the chamber, has been the use of a conical, perforated plate. An early example of such a plate is disclosed in U.S. Pat. No. 460,054 which discloses two concentric truncated "thimbles" having perforated walls. A single truncated cone mounted concentrically within a perforated cylinder is disclosed in U.S. Pat. No. 4,095,933. Two truncated cones, joined at their bases and mounted in series, are disclosed in U.S. Pat. No. 4,113,425. A single truncated cone is disclosed in U.S. Pat. No. 4,963,089, the smaller diameter of the truncated cone being oriented toward the upstream end of the combustion chamber. The proper and definitive use of conical baffling, including shape, wall perforation patterns, cone orientation, number of cones, and cone placement is not disclosed in these aforementioned references, which instead rely on empirical or unarticulated rules of implementation.
Prior to the introduction of fuel to the combustion chamber, the fuel is typically distributed and atomized in order to promote thorough combustion. For example, U.S. Pat. No. 2,396,306 discloses the use of a fuel pipeline which enters a continuously rotating head, or "slinger," thereby throwing fuel in a radially outward direction. A disc mounted coaxially with the fuel distribution head rotates at the same rate. Fan blades mounted to the disc tend to atomize the fuel as it exits the rotating head. U.S. Pat. No. 2,227,195 discloses a radially displaced "vaporizing ring" extending along the inner peripheral edge of the combustion chamber. The ring is formed as a series of tapering serrations which tend to break up the fuel upon physical impact therewith. U.S. Pat. No. 5,236,350 discloses a rotating fuel distribution nozzle in which a pair of fuel jets is routed into each of a series of outwardly oriented channels which sling fuel into the combustion chamber. Each of these references illustrate the complexity and uncertainty which characterize prior art fuel atomization efforts.
Typically, portable kerosene heaters of the type embodied in the present invention are mounted on a trailer or chassis which also serves, at least in pan, as the fuel tank. The fuel tank is therefore below the fuel distribution, ignitor and combustion areas of the heater. Some means must be provided for elevating the fuel into the region of the ignitor, as well as atomizing and metering the fuel to the ignitor in controlled amounts. An example of such a burner is disclosed in U.S. Pat. No. 4,998,878, which utilizes, in addition to other components a fuel delivery system having a discrete fuel filter, fuel pump, solenoid valve, nozzle, fuel control valve, shutter control valve, needle valve, fuel inlet and outlet lines and apertures, and additionally, a fuel flow control system further including, among other components a piston, piston rod, shutter plate, actuator bar and actuator rod. The complexity of such an arrangement is expensive and prone to failure and misadjustment.
Other components of a portable kerosene heater include a fan upstream of the fuel slinger and ignitor, the fan tending to direct the fuel toward the ignitor, and once ignited, to urge the flame downstream through tile combustion chamber. An example of such an axial flow fan is disclosed in U.S. Pat. No. 5,226,783, which utilizes a plurality of "centrifugal elements" to inhibit countercurrent and circulation flow. The airstream lines disclosed achieve a particular result, but are not suitable for use in a burner application insofar as the requirements of a smaller hub and airflow in the region of the fuel nozzle and ignitor are not addressed.
A final feature present in prior art fuel burners is a safety device to insure that the ignitor is deactivated when tile flame is extinguished for any reason. An example of such a device is disclosed in U.S. Pat. No. 4,298,335, which includes a start up circuit, heater timer, coupling circuit, valve control circuit and a flame sensing circuit which relies on rectification of current by tile flame itself. The complexity of this arrangement is expensive and relies on the presence of a flame, rather than the sensing of the relevant combustion temperature, to control its operation.